This invention relates to a clutch and brake mechanism and in particular to such a mechanism for controlling the rotation of a print drum of a postal franking machine.
Postal franking machines are known in which printing of the franking impression is effected by a printing plate carried by a rotatable print drum. The drum usually has a fixed printing plate for printing fixed information such as the authorized pattern of the franking impression and settable print wheels which project through an aperture in the drum for printing the variable information such as the value of the postage charge and the date of franking. When a user desires to frank a mail item, the value of the required postage charge is set into the machine by operation of levers or push buttons and this causes the print wheels to be set to print the required value of postage charge in the franking impression. After setting of the print wheels, the print drum is caused to rotate through a single revolution so as to roll the printing plate and print wheels against the surface of mail item to be franked. The franking machine is provided with means to carry out accounting functions so as to account for the value of postage used. In pre-payment systems the accounting means stores a value of credit available for use in franking and this credit value is decremented by the value of postage charge used in each franking operation. Prior to rotating the print drum to print a franking impression, the accounting means carries out a check to determine whether sufficient credit is available for the value of franking to which the machine has been set. If there is sufficient credit, the print drum is released for rotation and is driven, usually by means of a single revolution clutch mechanism, to print the franking impression. Upon completion of the single revolution in which printing is effected the drum is prevented from further rotation by means of locking mechanism and is only released for rotation when the next franking operation is initiated. The print drum is locked against rotation in order to prevent manual rotation of the drum to effect printing of a franking impression without a corresponding accounting by the accounting means in respect of use of the value of the frank.
The single revolution clutch for driving the print drum and the locking means for preventing rotation of the print drum are usually relatively complex mechanisms and when the accounting means comprises electronic circuits it is necessary to provide electro-mechanical actuators to control operation of the mechanisms. Furthermore the clutches and locking devices tend to snatch in and subject the mechanism to relatively high acceleration and deceleration forces which lead to mechanical wear and failure. These can frequently become limiting factors to the performance and reliability attainable in franking machines.